1977 was the first year that Rawlings manufactured major league baseballs for both leagues but the first World Series with it's own commemorative ball was 1978 with a ball stamped with "75th World Series" on the bottom panel.
They apparently used regular National and American League baseballs in the 1977 World Series with the home team supplying the baseballs used in each game. Maybe the Yankees still had a few Spalding balls left over from 1976? In any case, a ball from game 6 at Yankee Stadium should be a American League Lee MacPhail ball. If your friend's ball is a National League Charles Feeney ball then the story has less of a leg to stand on.
A couple of years ago, a ball was auctioned off as one of Reggie Jackson's three home runs in game 6 of the '77 Series. It was a Rawlings American League ball.
http://sports.ha.com/itm/baseball-co....s#74547454501
What puzzles me about the Jackson ball is there seems to be 3 dots under the Rawlings label which most collectors would date to 1978. As you can see from the 1977 ball below, it has two dots. Ball collectors generally believe the dots was Rawlings method of dating baseballs, adding a dot each year until they reached 5 dots, then a new cycle would begin with a single dot again. Maybe they used balls manufactured for 1978 in the 1977 Series? Don't know.